The present invention relates to a direct-coupling control method for an automatic transmission equipped with a driving force transmission which has input and output-side driving force transmitting elements normally coupled to each other through hydraulic oil for torque transmission between them. It further has a direct-coupling mechanism through which, if necessary, the transmitting elements are directly coupled to each other in a manner permitting occurrence of slip therebetween. More particularly, it relates to a direct-coupling control method which is capable of suppressing vibration caused by deteriorated hydraulic oil.
An automatic transmission mounted on a motor vehicle has a driving force transmission, e.g., a torque converter, installed between an internal combustion engine and a gear transmission. It is arranged to transmit the driving force of the engine to the gear transmission via the torque converter. The torque converter is provided with an impeller (input-side driving force transmitting element) coupled to a crankshaft of the engine for rotation in unison with a front cover of the converter, a turbine (output-side driving force transmitting element) coupled to an output shaft of the converter for torque transmission between itself and the impeller through hydraulic oil, and a direct-coupling mechanism.
The direct-coupling mechanism operates to control an amount of slip between the crankshaft and the impeller, i.e., slippage (a rotational speed difference) between the turbine and the impeller. In a predetermined operating area of the internal combustion engine, the direct-coupling mechanism serves to reduce the slip amount to zero, so as to eliminate energy loss caused by the slippage between the turbine and the impeller, thereby improving fuel efficiency. The larger the direct-coupling operating area wherein the slip amount is controlled to zero, the better the fuel efficiency will be. For this reason, even in a particular operating area of the internal combustion engine such as a low engine speed area thereof, there has been a demand for direct-coupling operation in which the slip amount is controlled to zero.
However, if the slip amount is reduced completely to zero in the particular operating area such as the low engine speed area, then body vibration called "confined noise" is generated, posing a problem of uncomfortable drive feeling. Thus, it has been proposed to employ minute-slip direct-coupling control that is designed to allow slight slippage rather than controlling the slip amount completely to zero in the particular operating area such as the low engine speed area.
In such minute-slip direct-coupling control, it is important to ensure stable control of the direct-coupling mechanism in an area with an extremely small slip amount .DELTA.S. In this connection, it is necessary to use hydraulic oil having such a characteristic that a friction factor .mu. of engaging elements of the direct-coupling mechanism decreases as the slip amount .DELTA.S decreases as shown by the solid line in FIG. 1. To obtain such a characteristic, a special additive is added to the hydraulic oil. However, when the hydraulic oil deteriorates after extended use so that the friction factor suddenly increases in the minuteslippage area as shown by the broken line in FIG. 1, the minute-slip control becomes difficult, presenting problems such that hunting occurs in the slip amount, and direct-coupling vibration takes place.